Pulp washing is a key operation in the chemical pulping line. There are many different types of washing and dewatering apparatuses available, some of which are based on washing by pressing the pulp such that fluid is removed.
A well-known type of wash press has two co-operating cylindrical press rolls, arranged with their center of rotation in the same horizontal plane. The outer surface of each press roll is perforated and, during operation, cellulose pulp is input to a restricted space between the perforated roll surface and a restriction member, such as a vat, whereby a pulp web is formed on the perforated roll surface. The press rolls are arranged to rotate in opposite directions so as to transport the respective pulp webs in the direction of rotation to be pressed in a so-called pinch or nip where the distance between the press rolls is smallest.
The fluid removed from the pulp (i.e. the filtrate) passes through the perforated roll surface in a radially inward direction and can for example be transported to the ends of the respective press rolls by means of axial filtrate channels. There is normally a filtrate tank arranged in connection with the wash press to collect all filtrate resulting from the washing in the press. There is often a supply of washing liquid to the wash press, and since the washing liquid displaces fluid in the pulp, the washing principle will in such a case be a combination of dewatering, displacement and pressing.
A wash press of the described general type is disclosed in U.S. Pat. No. 3,980,518, for example.
Another example of a wash press of the above-described general type is the wash press disclosed in EP 1 035 250. The objective of this wash press is to improve the total dewatering and this particular wash press has a vat design in which the vat is converging towards the outer surface of the press roll, in the direction of rotation of the press roll. The vat is arranged to enclose the outer surface of the press roll from a pulp inflow chamber placed in the region of the press roll's highest point and further around at least 230° of the outer surface's circumference, so that the pulp web formed is constrained to run between the outer surface and the vat around at least 230° of the circumference while being subjected to a converging vat, before the fibrous web reaches the final pinch between the press rolls.
A problem associated with a wash press with a long converging enclosed area is the tendency of plugging of the pulp suspension in the confined area between the vat and the outer surface of the press roll. Plugging of pulp suspension leads to undesired stops in the operation of the wash press with time-consuming cleaning operations and loss of production as a result. Another problem associated with such a wash press is that the pressure profile associated with the converging space leads to a rather tough treatment of the pulp suspension confined in the space between the outer surface of the press roll and the surrounding vat. The pressure created by the converging geometry forces not only fluid from the fiber suspension, but also fibers, through the perforations of the outer surface of the press roll. This leads to losses of valuable fibers, which results in lower production of pulp coming out of the washing/dewatering operation. Moreover, fibers in the filtrate complicate the handling of the filtrate and may demand external equipment in order to recover the fibers from the filtrate. This is especially the case if the filtrate is to be purged to external treatment or to a recipient. Yet another problem with wash presses with converging vat profiles is the build up of friction forces acting on the vat structure, which calls for a strong load supporting structure for the vat structure, inducing increased costs for the load supporting structure.
Accordingly, there is a need for a wash press reducing the problems associated with a wash press of the kind described above.
One of the objects of the present invention is to provide an improved arrangement for washing cellulose pulp. A specific object is to achieve improved runnability of a wash press with cylindrical press rolls where a rather large part of the circumference of the press rolls is enclosed by a vat. Another object of the present invention is to reduce the fiber content in the output flow of filtrate from the wash press.
Still other objects of the present invention are to enable wash presses in which the fiber suspension being treated is subjected to more lenient conditions, implicating less damage to the fibers in the suspension and to enable wash presses which are less sensitive to variations in terms of for example concentration and flow of the pulp suspension fed into the wash press.